1. Field of the Invention
The invention relates to vehicle brake assemblies and, more particularly, to parking brake assemblies designed to prevent vehicle wheels from rotating even when the vehicle is positioned on a steep grade.
2. Discussion of the Related Art
Parking brake assemblies are well known for preventing wheels of motor vehicles from rotating when the vehicle is not in operation. Parking brake assemblies also traditionally function as emergency brakes to automatically brake the vehicle wheels in the event of an externally transmitted emergency signal, fluid pressure loss, etc. The typical parking brake assembly is a spring applied/fluid released disc brake assembly having one or more friction pads engaging a rotor to brake the vehicle. Because friction brakes rely on friction for braking rather than a positive mechanical locking, the brake pads will slip on the rotor once a designated torque threshold is exceeded. This torque threshold is a function of the number and size of the friction pads, the type of friction material employed for the pads, and the spring force acting on the pads. The most common way to increase this torque threshold is to increase the spring force acting on the pads. However, the hydraulic or pneumatic pressure required to overcome the spring force must necessarily increase with the applied spring force--requiring more energy to pressurize the fluid and requiring the use of relatively expensive high pressure pumps, fittings, hoses, etc.
The torque threshold problem associated with friction brakes is especially evident with trench rollers and other industrial vehicles which often must be braked on steep grades. Indeed, government regulations and industry requirements demand that parking brakes for these types of vehicles apply sufficient torque to brake the vehicle on a grade which is at least as steep as that up which the vehicle is capable of climbing. Friction brakes traditionally used in this type of vehicle and exhibiting an acceptably high slip threshold have a relatively large physical size, require high release pressures on the order of 300 psi (2 MPa), and are prohibitively costly. Friction brakes also are prone to sticking, are prone to wear, and require frequent maintenance or replacement.